Plant Hormones

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Plant Hormones

• AP Biology Ch. 39

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What are Hormones?

• Chemical signals that coordinate the various
  parts of an organism
• Chemicals are made in one region and are target
  for some other region of the organism

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The Discovery of Plant Hormones

• Plant hormones were discovered as scientists were
  studying how it is that plants grow towards light
• Phototropism growth of a shoot towards light
• Click here to see movies
• Charles Darwin (and his son)

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Darwins Experiments with Phototropism

• Coleoptile term for the sheath that encloses a
  grass seedling
• Studied growth of the coleoptile in different
  conditions
• Darkness grew straight
• Illuminated uniformly from all sides grew
  straight
• Illuminated from one side only grew towards the
  lighted side

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Question What Causes the Coleoptile to Bend
towards Light?

• Hypothesis cells on the darker side of the
  coleoptile elongate faster than those on the
  lighted side. This causes the coleoptile to bend
  toward light.
• How does this happen?

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Darwins Ideas

• The part of the coleoptile responsible for
  sensing light is the TIP.
• Growth response that caused curvature of the
  coleoptile was BELOW the tip.
• Hypothesis some signal was transmitted downward
  from the tip

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Diagrams of Experiments
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Testing Darwins Hypothesis

• Peter Boysen-Jensen
• Tip was separated from the coleoptile
• Control treatment A gelatin block separated the
  tip form the lower parts of the plant
• The gelatin block allowed the plant to be cut as
  it would be in the experimental treatment, but
  still allowed the chemicals from the tip to pass
  down
• Resulted in curvature as normal

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Testing Darwins Hypothesis

• Experimental treatment
• An impermeable barrier was placed between the
  coleoptile tips and the lower parts of the plants
• Prevented the chemicals made at the tip from
  moving down the plant
• Result Curvature growth did not occur

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Diagrams of Experiments
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Wents Experiments

• Extracted the chemical messenger from the
  coleoptile tip
• Removed the tip and allowed it to diffuse onto a
  piece of agar
• Removed and discarded growing tip from other
  coleoptile seedlings
• Placed the agar block evenly centered onto the
  decapitated seedlings
• They grew straight
• Placed the agar block Uncentered onto other
  decapitated seedlings
• Their growth caused them to curve AWAY from the
  side with the agar block

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Wents Experiments
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Wents Conclusions

• The chemical in the tip stimulated growth as it
  passed down the coleoptile
• The coleoptile curved tward light because of a
  HIGHER concentration of the growth-promoting
  chemical on the DARKER side of the coleoptile
• Went named the chemical messenger that he studied
  AUXIN

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Tropisms

• Growth responses that result in curvatures of
  whole plant organs toward or away from some
  stimulus.
• Mechanism
• Elongation of cells on the OPPOSITE side of the
  organ region that is receiving the stimulus
• Stimulii
• Gravity
• Light
• Touch

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How Does Auxin Stimulate Growth?

• Causes cell walls to become looser and more
  malleable. Then they can be expanded/elongated

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List of Plant Hormones

• Auxin IAA (indoleacetic acid)
• Found
• Meristems of apical buds young leaves
• Major Functions
• stimulation of stem elongation
• Allows for stem cell walls to become pliable so
  that addition of water will cause elongation
• Root growth differentiation, branching
• Allows formation of adventitious roots at cut
  base of stem
• Apical dominance
• Growth of a stem occurs only at the tip unless
  the tip is cut off
• Absence of auxin from tip will allow lateral buds
  to emerge
• This is why we prune

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List of Plant Hormones

• Auxin (IAA) cont.
• Found
• Embryos within seeds
• Major Functions
• Stimulate growth of fruit from ovary

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List of Plant Hormones

• Cytokinins
• Found
• In actively growing tissues especially roots.
  Transported by xylem to other places
• Major function
• Role in division and differentiation of plant
  cells
• Work with auxins to control plant growth
• Plant tissue treated with auxin w/o cytokiinin
  cells will grow but not divide
• Control of apical dominance
• Cytokinins from roots can counter action of auxin
• Anti-aging hormones
• May inhibit protein breakdown by stimulating
  protein synthesis
• Slow deterioration of leaves used by florists

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List of Plant Hormones

• Gibberellins
• Found in
• Apical meristems young leaves/embryos
• Major function
• Simulates growth in leaf and stem
• Both elongation AND cell division auxin link
• Stem bolting
• Rapid elongation of a stem flower stalk
• Fruit growth
• Grapes are sprayed with gib to cause them to grow
  larger and further apart
• Germination of seeds
• After water is imbibed, gibberellins are released
  from embryo to signal break from dormancy

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List of Plant Hormones

• Abscisic Acid (ABA)
• Found in
• Leaves, stems, roots
• Seeds, green fruit
• Major function
• Slow down growth
• Dormancy for overwintering
• Suspends primary and secondary growth
• Promotes abscision of leaves (falling off)
• In seeds inhibits growth until ABA can be
  overcome or diminished by favorable conditions
• Heavy rain may wash out ABA
• Light may degrade
• Increased gib to ABA ratio may determine
  germination
• growth
• Stress hormone
• When a plant wilts, ABA accumulates causing
  stomata to close

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List of Plant Hormones

• Ethylene
• Found in
• Tissues of ripening fruit
• Nodes of stems
• Ageing leaves and flowers
• Major functions
• Changes of ovary to become fruit
• Degradation of cell walls softening
• Dropping from plant
• Leaf abscission
• Loss of leaves to prevent water loss
• Tissue at base of petiole dies
• Senescence (aging)
• Autumn leaves withering flowers

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Phototropism

• Cells elongate on the side of the stem that is
  in the DARK
• This occurs due to an ASSYMETRICAL distribution
  of auxin from the shoot tip
• Click here to see movies

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Gravitropism

• Growth in response to gravity
• Allows roots to know which way is DOWN and to
  grow in that direction
• Allows shoots to know which way is UP and to
  grow in that direction
• Hormone is auxin
• Mechanism is similar to phototropism, but
  stimulus is gravity
• Click here to see movies

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Thigmotropism

• Growth in response to touch
• Seen in vines that know where a fence post is
• Curvature in growth allows them to cling to the
  post and move up towards light

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Turgor Movements

• Compared to growth responses to stimulii, these
  movements are rapid.
• Turgor movements are also reversible
• Example
• Folding of leaves in response to touch
• Response is due to a rapid loss of turgor
  pressure by cells within speacialized organs at
  the joints of leaves

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Turgor Movements

• Possible advantages
• Reduce surface area in wind preventing
  dessication
• Expose thorns if an animal is trying to eat leaves

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Control of Daily and Seasonal Responses

• Plants may respond to stimulli within the course
  of a day or as seasons change
• Modes of control are through biological clocks
  that control circadian rhythms
• Circadian rhythm physiological cycle with a
  frequency of 24 hours
• Circadian rhythms persist even when cues of light
  and dark are removed
• However, the rhythms tend to get off schedule
• Animals have circadian rhythms, too.

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Photoperiodism

• Synchronizes plant responses to changes of
  seasons
• A physiological response to photoperiod
• Photoperiod relative length of day and night

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Photoperiodism

• Short day plant
• Light period shorter than some critical length
  causes flowering
• Plants usually flower in late summer, fall/winter
• Long day plant
• Light period longer that some critical length
  causes flowering
• Plants usually flower in spring/early summer
• Day-neutral
• Plants flower upon reaching a certain stage of
  maturity regardless of day length

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Photoperiodism and NIGHT length

• Cocklebur flowers
• If daytime portion of cycle is broken with a
  brief exposure to darkness, nothing happens to
  the flowering process
• HOWEVERif the night portion of the cycle is
  broken with a brief exposure to light, then
  flowering will NOT occur
• Thus, in order to flower, the cocklebur requires
  no specific day length, but DOES require at least
  8 hours of continuous darkness.

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Photoperiodism and NIGHT length

• Short day plants are really long night plants
• Long day plants are really short night plants
• BOTTOM LINE
• Photoperiodic responses depend on a critical
  night length.

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Use of photoperiodism in the floral industry

• Chrysanthemums would normally bloom in May, but
  generally they are sold and bought in the fall.
• What do florists do to keep them from blooming in
  May?
• They continually interrupt the mums night with a
  flash of light to delay blooming. When they want
  the plants to bloom, they stop interrupting the
  night photoperiod.

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Phytochromes

• Chemicals that function as photoreceptors in
  plants and allow plants to measure photoperiod

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Phytochromes

• Red 660nm
• Wavelength of light that is most effective at
  interrupting the critical night length of a short
  day (long night) plant.
• Exposure at night will cause the plant NOT to
  flower
• HOWEVER, if this light briefly interrupts the
  night of a long day (short night) plant, the
  plant will flower
• The red flash will shorten the plants perception
  of night length

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Phytochromes

• The shortening of night length can be negated by
  providing a flash of light at 730nm wavelength.
• This is called the far-red part of the spectrum

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Phytochromes

• If RED LIGHT (R) flashed during a dark period is
  followed by a flash of FAR-RED light (FR) the
  plant perceives NO INTERRUPTION of night.
• Each wavelength of light negates the effect of
  the one that recedes it in a series of flashes

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Phytochromes

• The name given to the photoreceptor that is
  responsible for the reversible effects of red and
  far-red light is phytochrome
• Phytochrome a light absorbing protein
• 2 forms
• Pr absorbs red light
• Pfr absorbs far red light
• The two forms are photoreversible
• When Pr is exposed to red, it becomes Pfr
• When Pfr is exposed to far red, it becomes Pr

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Phytochromes

• The Pr lt-gt Pfr interconversion acts as a
  switching mechanism that controls various events
  in the life of a plant.

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Phytochrome Example 1

• Pfr and Pr are in equilibrium in daylight
• Pr accumulates at night
• Because there is no sunlight to make the
  conversion from Pr to Pfr
• At daybreak, sunlight rapidly converts Pr to Pfr
  and vice versa. Equilibrium is achieved
  relatively quickly.

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Phytochrome Example 1

• Night length is responsible for resetting the
  circadian rhythm clock
• If daylight is interrupted with dark there is no
  effect
• If dark is interrupted with flashes of red or
  far-red the clock can be affected
• Red-light shortens night length
• Because it converts Pr to Pfr which would not
  normally accumulate at night
• Far-red light restores as though night was not
  broken
• Because far red light flashes convert Pfr back to
  Pr

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Phytochrome Example 2

• Plants synthesize phytochrome as Pr
• If left in the dark, nothing happens to this
  pigment
• If the pigment is illuminated with sunlight, Pr
  changes to Pfr
• Thus the plant can detect the presence of
  sunlight

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Phytochrome example 2

• Since sunlight includes both red and far-red
  radiation, the amounts of Pr and Pfr should be in
  equilibrium during the course of a day in bright
  sun
• IF shade of larger trees were to block sunlight
  from a smaller tree, the radiation that is most
  blocked by the forest canopy is red (not far red)
• This means that the pigments in the plant would
  be converted to Pr
• This cue would stimulate the plant to grow taller.

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Phytochrome Example 2

• If ample sunlight were available, the reverse
  would happen
• Pfr proportions would increase and the plant
  would sense that it was in sun.
• It would be cued to branch and vertical growth
  would be inhibited